The presence of unburned material in the ejecta of normal Type Ia supernovae (SNe Ia) is investigated using early-time spectroscopy obtained by the Carnegie Supernova Project. The tell-tale signature of pristine material from a C+O white dwarf progenitor star is the presence of carbon, as oxygen is also a product of carbon burning. The most prominent carbon lines in optical spectra of SNe Ia are expected to arise from C II. We find that at least 30% of the objects in the sample show an absorption at 6300 Å which is attributed to C II λ6580. An alternative identification of this absorption as Hα is considered to be unlikely. These findings imply a larger incidence of carbon in SNe Ia ejecta than previously noted. We show how observational biases and physical conditions may hide the presence of weak C II lines, and account for the scarcity of previous carbon detections in the literature. This relatively large frequency of carbon detections has crucial implications on our understanding of the explosive process. Furthermore, the identification of the 6300 & 0197; absorptions as carbon would imply that unburned material is present at very low expansion velocities, merely 1000 km s–1 above the bulk of Si II. Based on spectral modeling, it is found that the detections are consistent with a mass of carbon of 10–3 to 10–2 M ☉. The presence of this material so deep in the ejecta would imply substantial mixing, which may be related to asymmetries of the flame propagation. Another possible explanation for the carbon absorptions may be the existence of clumps of unburned material along the line of sight. However, the uniformity of the relation between C II and Si II velocities is not consistent with such small-scale asymmetries. The spectroscopic and photometric properties of SNe Ia with and without carbon signatures are compared. A trend toward bluer color and lower luminosity at maximum light is found for objects which show carbon.

The authors acknowledge support from the NSF under grants AST-0306969, AST-0908886, AST-0607438, and AST-1008343, the World Premier International Research Center Initiative (WPI Initiative), MEXT, Japan, a Grant-in-Aid for Scientific Research for Young Scientists (23740175), and CONICYT through grants FONDECYT Regular 1060808, Centro de Astrofisica FONDAP 15010003, Centro BASAL CATA (PFB-06), and the Millennium Center for Supernova Science (P06-045-F).